The present invention relates to a phase shift mask, and more particularly to a method for fabricating a phase shift mask appropriately usable for fabrication of an ultra-fine semiconductor circuit structure.
Recently, semiconductor devices have been developed to exhibit a higher integration degree and a higher package density. In order to fabricate such semiconductor devices, a photomask having a more fine line width has been used. Otherwise, specific modified fabrication techniques have been proposed.
Generally, photolithography is the technique for forming an image pattern by transmitting light of a certain wavelength such as ultraviolet rays to the surface of a photoresist coated on a semiconductor substrate through a photomask. A photomask of the general type includes a photoshield pattern and a transmission pattern to achieve a selective light exposure. When such a photomask has an increased pattern density, a diffraction phenomenon may occur. For this reason, the photomask has a limitation on an improvement in resolution. This will be described in more detail.
Resolution and depth of focus obtained when a light exposure is carried out using the general type photomask can be expressed by the following equations (1) and (2): EQU R=k.sub.1 .multidot..sup..lambda. /NA (1)
where, R represents the resolution, k.sub.1 represents a proportional constant, .sup..lambda. represents a wavelength of source light used for the light exposure, and NA represents a numerical aperture, EQU D.O.F.=k.sub.2 -.sup..lambda. /(NA).sup.2 ( 2)
where, k.sub.2 represents a proportional constant and D.O.F. represents the depth of focus.
As the size of patterns is smaller, amplitudes of light on the wafer may be more difficult to be distinguished from one another because the difference between the crest and valley thereof is smaller due to a light diffraction occurring in adjacent patterns, as shown in FIGS. 1a to 1d. For this reason, there has been extensive research for improving the resolution by use of the phase shift lithography.
The phase shift lithography is the technique using a combination of a general transmission region adapted to transmit ejected light without any interference and an 180.degree.-phase-shifted transmission region made of a phase shift material, as each transmission pattern disposed between adjacent photoshield patterns. This technique can reduce the light diffraction problem because each photoshield pattern region generates a diffraction offset to that of each transmission pattern region adjacent thereto.
Accordingly, it is possible to form a pattern image similar to the mask image by sharply modulating the intensity of light. In order to transfer patterns exhibiting a high complexity, various lithography techniques have been developed.
As the phase shift mask, a Levenson type mask including a transmission film provided at one of neighboring transmission regions and adapted to shift the phase of light, and an edge emphasis type mask adapted to achieve a phase shift at edges of two different transmission regions have been used to reduce the photo-sensitivity.
Now, examples of conventional phase shift masks will be described, in conjunction with FIGS. 2a to 2h and FIGS. 3a to 3g.
FIGS. 2a to 2h are sectional views respectively illustrating fabrication of a conventional edge emphasis type phase shift mask.
In accordance with the illustrated fabrication method, a chromium layer 2 as a photoshield metal layer is first deposited over a transparent substrate 1, as shown in FIG. 2a. Over the chromium layer 2, a first photoresist film 3 is deposited.
Thereafter, photoshield pattern regions are defined at the first photoresist film 3 by selectively exposing the first photoresist film 3 to electron beams, as shown in FIG. 2b. The chromium layer 2 is then selectively removed such that only its portions respectively disposed at the photoshield pattern regions remain, as shown in FIG. 2c.
As shown in FIG. 2d, the remaining portions of the first photoresist film 3 are completely removed. Over the entire exposed surface of the resulting structure, a second photoresist film 4 is then deposited to a predetermined large thickness. Thereafter, the resulting structure is subjected to a back light exposure at the lower surface of the substrate 1 and then to a development, thereby causing the second photoresist film 4 to be patterned such that only its portions each disposed between adjacent photoshield pattern regions remain, as shown in FIG. 2e.
As shown in FIG. 2f, a phase shift layer 5 including, for example, a silicon oxide film is deposited over the entire exposed surface of the resulting structure. The phase shift layer 5 has a discontinuous structure because the patterned second photoresist film 4 has a large thickness.
Subsequently, the remaining portions of the second photoresist film 4 and portions of the phase shift layer 5 disposed over the remaining portions of the second photoresist film 1 are selectively removed by use of a lift-off process, as shown in FIG. 2g.
Using a wet etch process, the remaining portions of the chromium layer 2 respectively disposed at the photoshield regions are then over-etched, as shown in FIG. 2h.
On the other hand, FIGS. 3a to 3g are sectional views respectively illustrating fabrication of a conventional phase shift mask of the spatial frequency modulation type. In FIGS. 3a to 3g, elements corresponding to those in FIGS. 2a to 2h are denoted by the same reference numerals.
In accordance with the fabrication method, a chromium layer 2 and a first photoresist film 3 are sequentially deposited over a transparent substrate 1, as shown in FIG. 3a. The first photoresist film 3 is selectively exposed to electron beams at its portions corresponding to transmission regions and then subjected to development. That is, the first photoresist film 3 is patterned such that its portions respectively disposed at the transmission regions remain, as shown in FIG. 3b.
Using the patterned first photoresist film 3 as a mask, the exposed portions of chromium layer 2 are selectively removed, as shown in FIG. 3c.
Thereafter, the remaining portions of the first photoresist film 3 are completely removed, as shown in FIG. 3d. Over the entire exposed surface of the resulting structure, a second photoresist film 4 is thickly coated. Subsequently, the second photoresist film 4 is selectively exposed to electron beams at its portions respectively corresponding to the transmission regions. In this case, the electron beam exposure is not carried out for all the transmission regions, but the electron beam exposure is carried out for every other transmission region.
The second photoresist film 4 is then subjected to development, thereby removing its portions exposed to the electron beams, as shown in FIG. 3e.
Over the entire exposed surface of the resulting structure, a phase shift layer 5 including, for example, a silicon oxide (SiO.sub.2) film is deposited, as shown in FIG. 3f. Similar to the case of FIGS. 2a to 2h, the phase shift layer 5 has a discontinuous structure because the patterned second photoresist film 4 is so thickly deposited as to form high steps.
Subsequently, the remaining portions of the second photoresist film 4 and portions of the phase shift layer 5 disposed over the remaining portions of the second photoresist film 4 are selectively removed by use of the lift-off process, as shown in FIG. 3g. Thus, a phase shift mask of the spatial frequency modulation type is obtained.
The conventional phase shift mask of the edge emphasis type achieves a phase shift at edges of photoshield patterns whereas the conventional phase shift mask of the spatial frequency modulation type achieves a phase shift at one of the neighboring transmission regions.
FIG. 4 is a plan view illustrating the layout of a conventional phase shift mask of the outrigger sub-resolution type. As shown in FIG. 4, this phase shift mask has fine sub-patterns B and B' disposed in the vicinity of or around each transmission region A at which an actual pattern is to be formed. The fine sub-patterns B and B' serve to prevent an interference phenomenon occurring due to a light diffraction generated in a light exposure.
Fabrication of this phase shift mask of the outrigger sub-resolution type will now be described.
For preventing the interference phenomenon occurring due to the light diffraction by use of the sub-patterns, the width ratio between the transmission region A corresponding to the actual pattern and the fine sub-patterns B and B' disposed around the transmission region A should be 3:1. Also, each of photoshield regions C and C' respectively disposed between the transmission region A and the fine sub-pattern B' should have the same width as the transmission region A. Therefore, the width ratio among the regions B', C', A, C and B must be 1:3:3:3:1. At this width ratio, an optical characteristic can be obtained for offsetting the light diffraction generated at edges of each transmission region A corresponding to the actual pattern.
In a typical fabrication of such a phase shift mask of the outrigger sub-resolution type, each fine sub-pattern is formed to have the width of about 0.15 .mu.m when the transmission region at which the actual pattern is to be formed has the width of about 0.4 .mu.m. Since the pattern width is very small as mentioned above, patterning of the sub-fine patterns should be carried out using electron beams.
In order to fabricate the phase shift mask, a chromium layer as a photoshield layer and a photoresist film for electron beams are deposited over a transparent glass substrate. Thereafter, electron beams are selectively ejected onto each transmission region A at which an actual pattern is to be formed and each of fine sub-patterns B and B' disposed in the vicinity of the transmission region A. The photoresist film is then subjected to a development to define transmission regions. Using the photoresist film as a mask, the chromium layer is selectively removed, thereby forming a phase shift mask of the outrigger sub-resolution type.
However, the above-mentioned phase shift masks have the following problems.
First, in the case of the phase shift mask of the edge emphasis type, an under-cut phenomenon occurs upon isotropically wet etching edges of photoshield regions. As a result, it is difficult to expect an accurate phase shift effect.
Second, it is difficult to correct a defect generated at a mask because the patterns of the phase shift layer and photoshield metal layer formed on the transparent substrate have a reverse critical dimension structure.
Third, although the phase shift mask of the spatial frequency modulation type provides a better phase shift effect over the phase shift mask of the edge emphasis type, it requires a more complex process involving an additional photomasking step because phase shift layers are selectively formed in transmission regions.
Fourth, in the case of the phase shift mask of the outrigger sub-resolution type, the regions B', C', A, C and B including each transmission region A at which an actual pattern is to be formed, fine sub-patterns B and B' respectively disposed in opposite sides of the transmission region A, and photoshield regions C and C' respectively disposed between the transmission region A and the fine sub-pattern B and between the transmission region A and the fine sub-pattern B' must have a certain width ratio of, for example, 1:3:3:3:1. However, it is difficult to accurately obtain the pattern width ratio because attraction or repulsion occurs among electrons charged up in the photoresist film for electron beams upon the electron beam exposure, due to the small pattern width. Furthermore, there are disadvantages of an increased manufacture cost and a degraded yield.